Experimental autoimmune encephalomyelitis (EAE) is a disease that can be induced in genetically susceptible animals by immunizing them against the basic protein (BP) of myelin. Recently, we have succeeded in isolating and growing as cell lines rat T lymphocytes specifically reactive against BP. The unizue feature of these cell lines is that they are functional both in mediating EAE and in protecting against EAE. Intravenous inoculation of relatively small numbers of cells of some of these lines, or of clones derived from them, produces EAE in syngeneic Lewis rats within a few days. Moreover, a single intravenous injection of attenuated cells of these lines leads to resistance in about 65% of recipient rats to induction of EAE by subsequent active immunization against BP. The proposed research has three long-term objectives: 1) to learn how our anti-BP T cell lines and clones mediate EAE, 2) to elucidate the mechanism by which attenuated line cells vaccinate against active EAE, and 3) to investigate whether line cells can also be used to treat ongoing, chronic EAE.